The Bureau of Alcohol, Taxes and Firearms (ATF) has officially declined to impose regulations on devices which it has designated as an "accessory". Specifically, those federal employees would not touch the "Bump Stock" issue with the proverbial 10 foot pole!
ATF Discontinues Accessory Classifications – Prince Law Offices Blog: if an individual or company submits an accessory to ATF for classification and it is not attached to a firearm, they won’t be rendering any decisions on it. Some readers inquired what would happen in a case where the accessory made the firearm subject to the control of the NFA. In the past, ATF has returned those items to individuals or companies that hold the appropriate type of SOT. If the individual or company lacks the appropriate SOT, ATF has not returned the item. To the best of my knowledge, ATF has not previously charged any individuals with GCA or NFA violations for seeking a determination.You may be forgiven for considering ATF to be the one Federal Department which is staffed by "people who know better ...: